The object of the invention is a device for cooling and recirculating the gas forming the protective atmosphere in the cooling chambers of furnaces for the continuous processing of metal strips, particularly for the continuous annealing of steel strips.
A device of this kind, so-called "jet cooler", comprises vertically arranged boxes which extend in the furnace interior between the successive vertically extending parallel strip sections, substantially over the whole length thereof, i.e. throughout the height of the furnace, and which have their walls turned toward the adjacent strip sections provided with perforations, to the said boxes there being associated a protective gas-cooling and recirculating unit comprising a heat exchanger, and a fan that draws the protective gas from the furnace interior and cools it by causing the same to flow through the heat exchanger, and that blows this gas at a certain pressure into the boxes, so that the cooled protective gas flows out of the perforations in the boxes in form of jets directed to the facing surfaces of the adjacent strip sections, whereby it causes the cooling of the strip.
In a known device of the above-disclosed kind, a plurality of suction ports communicating with the furnace interior, and a plurality of delivery ports communicating with the interior of the perforated boxes, are provided in the furnace sidewall. To all the perforated boxes, or at least to a number of same, there is associated only one cooling and recirculating system with only one fan that through suitable tubes draws the protective gas from the suction ports and causes the same to flow through only one heat exchanger, so that it blows the cooled gas through suitable delivery tubes into the vertically arranged perforated boxes. The drawback of this known device resides in the fact that big volumes of protective gas will be displaced from the furnace interior through suction and delivery tubes into an external cooling and recirculating system, which increases the risk of this gas becoming polluted owing to any possible leaks in the tubes, or to any defect in their tightness. Moreover, the said known device has great overall dimensions at the furnace outside, and occupies a rather large floor space.
In another known device for cooling and recirculating the protective gas in the cooling chambers of furnaces for the continuous annealing of steel strips, instead of the perforated boxes extending vertically over the whole height of the furnace, a plurality of individual horizontally arranged boxes are provided between the parallel steel strip sections, and these boxes extend into the furnace transversely to the longitudinal direction of the said steel strip sections, and are distributed at predetermined intervals over the furnace height. Each one of these individual horizontally arranged boxes is tightly welded to the furnace sidewall and partly projects out of the furnace. Associated to each horizontally arranged individual box are a heat exchanger and a fan, preferably a centrifugal fan, which are housed in the box portion projecting out of the furnace. The opposite sidewalls of each horizontally arranged individual box, which are turned toward the two adjacent strip sections, are perforated. Moreover, each horizontally arranged box has a suction chamber containing the heat exchanger and communicating with the interior of the furnace through suction ports provided in the upper and/or the lower end of the horizontally arranged individual box. The fan of each horizontally arranged individual box draws the protective gas through the said suction ports and through the said suction chamber in the box, thus causing the said gas to flow through the heat exchanger, where the gas is cooled. The cooled protective gas is blown by the fan into the respective horizontally arranged individual box, and is caused to flow out through the perforations in said box, in form of cooling jets directed to the two adjacent strip sections.
This known device with a plurality of horizontally arranged individual boxes provided in a superposed aligned relation between every two successive vertically extending strip sections, has the drawback that the entire perforated useful area through which the cooled protective gas jets are directed to the strip, is only a fraction of the area directly facing the strip, owing to the vertical spacing apart of the superposed horizontally arranged individual boxes from one another. This remarkably decreases the useful cooling area and--with the available spaces being equal, proportionally decreases the installable cooling power. Therefore, this known device does not satisfactorily meet the requirements of the continuously operated modern furnaces, in which a specific high cooling power needs to be installed.
Another drawback of the said known device with horizontally arranged individual boxes resides in the fact that when one or more of the individual protective gas-cooling and recirculating units, respectively associated to the horizontally arranged individual boxes forming part of the same vertically extending file of these boxes, should stop owing to a failure, a local unbalance in the pressures acting on the strip might be produced whenever all the cooling and recirculating units in the next-adjacent vertically extending file of individual horizontally arranged boxes continue to be regularly operated, so that they tend to push the strip toward the individual horizontally arranged boxes comprising the cooling and recirculating units affected by a failure, and the strip is then caused to rub against the structure of these individual boxes. By such a rubbing, the surface of the strip may be damaged, and the final quality of the strip may be inevitably deteriorated. In order to reset a pressure balance and avoid the said rubbing of the strip, in the instance of a breakdown and a stopping of a certain number of cooling and recirculating units, respectively associated to the individual vertically arranged boxes in the same vertically extending file, the cooling and recirculating units of the facing horizontally extending boxes in the opposite file might be intentionally excluded, but this would determine a further important reduction in the overall cooling capacity.
A further drawback of the known device with individual horizontally arranged boxes arises from the fact that the heat exchanger of each protective gas-cooling and recirculating unit consists of a water-cooled finned tube group extending into, and out of the respective individual box with a recumbent U-shaped or horse shoe-shaped path. In this embodiment, the protective gas to be cooled tends to flow only through the arcuate inner part of the cooling tube group, while the rectilinear parts of said U-shaped cooling tube group are scarcely utilized, since they are situated in a fluid-dynamically calm zone of the suction chamber in the respective horizontally artranged individual box. Consequently, the actual efficiency of the heat exchanger is considerably reduced, whereby it produces, with the same overall dimensions, a further reduction in the specific cooling capacity.
The object of the invention is to eliminate the drawbacks of both of the above-disclosed kinds of known devices, and for this purpose the invention provides a cooling and recirculating device of the kind as disclosed at the beginning, substantially characterized in that a plurality of individual cooling and recirculating units are associated to each perforated vertically arranged box extending throughout the furnace height, and are distributed over the vertical extent of the respective box, and are attached to the outside of the furnace sidewalls.
In one preferred embodiment of the invention, each individual cooling and recirculating unit associated to one perforated vertically arranged box consists of a sealed case element, tightly attached to the outside of the furnace sidewall, and which is comparted into a suction chamber containing a heat exchanger and communicating with the interior of the furnace through the side of the respective perforated vertically arranged box, and into a delivery chamber containing a fan and communicating with the interior of the said perforated vertically arranged box, the suction inlet of the fan being connected to the said suction chamber, and the delivery outlet of the fan being in communication with the said delivery chamber.
By the cooling and recirculating device of the invention the advantages are maintained of the perforated vertically arranged boxes extending throughout the furnace height, but the drawbacks are eliminated of the known constructions using these vertically arranged boxes, since in place of only one cooling and recirculating system associated to all or to a number of the perforated vertically arranged boxes, a plurality of cooling and recirculating units are provided, which are associated to each perforated vertically arranged box and are distributed over the height of the same, and which contain each their own cooling and recirculating unit with a heat exchanger and a fan, and are applied to the outside of the furnace sidewall. Thus the piping of the protective gas over a long conveyance path outside of the furnace is avoided, and the risk of the said gas becoming polluted is then eliminated. Moreover, the device of the invention also has reduced overall dimensions as compared with the above-disclosed known construction having only one cooling and recirculating system, and these overall dimensions are however limited to the furnace sidewalls, and do not occupy any floor space. Consequently, the device of the invention also has a higher specific cooling capacity in relation to the occupied volumes.
With regard to the known constructions comprising perforated individual boxes horizontally arranged in a superposed aligned relation, the cooling and recirculating device of the invention affords the advantage of avoiding any local pressure unbalance in the case of a breakdown and a stopping of one or more of the individual cooling and recirculating units associated to the same vertically arranged perforated box. In fact, in such a case, the pressure decrease within the perforated vertically arranged box connected to the broken down cooling and recirculating unit or units is not limited to a localized relatively short section, but will spread through the respective vertically arranged perforated box, i.e. throughout the height of the furnace, so that a relatively small decrease of the total pressure in the said vertically arranged box will be produced. This decrease of the total pressure within the vertically arranged perforated box connected to the cooling and recirculating unit or units affected by a breakdown, is unlikely to become so great as to cause the strip to be rubbed against the structure of said box under the action of the protective gas jets being directed against the opposite face of the strip, and flowing out of the opposite respective vertically arranged perforated box having all its associated cooling and recirculating units working properly.
Besides this automatic balancing of the pressure in the interior of the perforated boxes, the device of the invention compared to the known construction with perforated horizontally arranged individual boxes, naturally affords all the advantages typical of the perforated vertically arranged boxes, particularly a higher cooling efficiency, since for the cooling protective gas jets directed against the strip it is taken advantage of all the furnace height, and not of only one portion thereof.
According to a further characteristic feature of the invention, the heat exchanger in each cooling and recirculating unit consists of a rectilinear water-cooling tubes group preferably vertically passed through the respective suction chamber. Thanks to this feature of the invention, the efficiency of the heat exchanger with equal overall dimensions is considerably improved with respect to the known construction with perforated horizontally arranged individual boxes and with a heat exchanger consisting of a recumbent U-shaped or horse shoe-shaped cooling tube group.
In comparison to the known constructions, the device of the invention furthermore affords the advantages of a simpler construction and a lower cost with reference to the attained performances, as well as the advantages of being very strong and of requiring practically no maintenance, or anyhow very little maintenance. The device of the invention is of a reduced weight, whereby it can be more easily disassembled and reassembled.